Good point keeping Java 7 support. I hadn't considered it but I think it might be possible with this one. I'm thinking something like:
try:
from java.time import LocalDate LocalTime LocalDataTime
java8_support = True
except ImportError:
java8_support = False
Then in the __tojava__ use the flag to switch on? Do you think that would work? Then there is the issue of writing tests without using the newer classes in Java still need to think about it.
Being able to register custom converters sounds like a nice idea, something that could be looked at.
I would be in favour of dropping Java 7 support but that's a much bigger question.